Understanding Different Census Geography Types
I received an excellent question on my previous blog post about the American Community Survey (ACS): Does the ACS, or Census Bureau more generally, provide statistics by urban area? The answer to that is a big fat YES. There are actually several different geography types that are specifically used to analyze urban areas and their surroundings. For now, I’ll focus on Core Based Statistical Areas (CBSAs) (I also call them metro areas more generally) because they fit a broader definition of what an urban area is.Core Based Statistical Areas (CBSAs) are comprised of at least one core area with a population of 10,000 or more and surrounding counties that exhibit a high degree of social and economic integration with the core area based on work commutes. CBSAs are great units of analysis if you are studying areas that are influenced by the economic and social activity of one or more cities or urban areas.
For instance, Washington, D.C. is a major employer hub for the surrounding counties in Maryland and Virginia – the proximity of the federal government makes these areas ideal for all kinds of companies, which require many employees, which then require more housing construction, which requires more public roads, which means more car buyers, which means more banking loans, and so on. Soon enough, it becomes difficult to distinguish where the domino effect of the city’s economic influence begins and ends.
The breakdown
Like counties, CBSAs have boundary definitions all of which are outlined by the Office of Management and Budget (OMB) and are updated approximately every decade. The last major definition update occurred in 2013, but the OMB is known to modify a CBSA in-between updates. Currently, there are two types of CBSAs that differ only in the population size of their core areas:
| CBSA type | Core area population requirement | Geographic building blocks |
|---|---|---|
| Metropolitan Statistical Area (MSA) | At least 50,000 people | Counties |
| Micropolitan Statistical Area | At least 10,000 but less than 50,000 people | Counties |
It’s important to note that a decade is a significant period of time and that CBSA boundary definitions are likely to change during this time. This can get tricky when analyzing one CBSA across more than 10 years because its boundary definitions are likely to have changed. You should keep this in mind when researching and analyzing CBSA statistics. Good data sources will provide the specific definition year for the CBSA’s boundaries used in a data set to avoid confusion.
How exactly do CBSA boundaries change over time?
CBSAs can gain or lose counties and sometimes new CBSAs are born and occasionally they are eliminated. Additionally, their names can change year-to-year based on the relative population of the largest cities. Again, be cautious when comparing CBSAs across any span of time greater than 10 years. For more information, visit the Census Bureau or visit our SAGE Stats Methodology page!
